The present invention relates to a silver halide photographic light-sensitive material having high sensitivity and high covering power, and more particularly to a silver halide photographic light-sensitive material for radiography use.
Silver halide photographic light-sensitive materials are required to have a high sensitivity and a high covering power; to be excellent in sharpness as well as in linearity of the shoulder and foot portions of the photographic characteristic curves thereof; and to be hardly desensitized when subjected to pressure. That is, with the recent progress of photographic technology, further increasing the photographic speed of silver halide photographic light-sensitive materials has been strongly demanded. For example, there have been demands for further increasing photographic speed so as to meet the respective needs for higher camera shutter speeds, for more rapidly processing color and black-and-white photographic papers, for electronically processing or simplifying the work in the graphic arts field, for attenuating X-ray exposure dose in the medical field, and so forth.
Above all, such demand is particularly strong in the medical radiographic field, which uses various techniques for the purpose of reducing the X-ray dose to which patients or radiographers are exposed. These techniques are essential for not merely attenuating the X-ray dose to which an individual is exposed but lessening the number of opportunities of group exposure.
In recent years, particularly with the increase in the number of medical X-ray checks, not only medical circles but also world opinion strongly call for reducing the exposure dose. In order to meet this demand, fluorescent intensifying paper screens, intensifying screens, fluorescent screens, and radiographic image intensifiers have been and are used, and efforts for improving these means and devices and increasing the photographic speed of photographic light-sensitive materials have been making remarkable progress in recent years. Also, closer medical examinations require highly accurate radiographic techniques. Since the larger the irradiating amount of X-rays, the higher the closeness of examinations, radiographic techniques using a large dose and also large-capacity X-ray generators have been developed. However, such radiographic techniques requiring a large X-ray dose, as stated above, can be rather counter to the need of reducing the exposure dose, and thus can be unacceptable. The radiographic field therefore needs a high photographic technology using a small X-ray dose. For this reason, it is indispensable to develop a photographic material capable of producing a high-quality image with a small X-ray dose, i.e., a further increased photographic speed-having photographic material.
On the other hand, future exhaustion of the resources of silver, principal raw material of silver halide photographic light-sensitive materials, is viewed with great anxiety. In addition, the price of silver can be quite unstable, for instance due to a steep rise in the price of petroleum. It is therefore desirable to reduce the amount of silver used for silver halide photographic light-sensitive materials as much as possible in order also to provide users with silver halide photographic light-sensitive materials at a stable price. To reduce the amount of silver, a covering power (optical density per unit amount of silver) improving technique is essential in addition to the photographic speed increasing technique.
Further, regarding the sharpness, for example, in radiographing affected parts of a living body, the radiographic image needs to be sharp and highly diagnosable also for the purpose of early detection of focuses and prevention of erroneous diagnoses; but conventional radiographic light-sensitive materials are not necessarily satisfactory for this purpose. An example of this is cerebroangiography, a recent widely-spread radiographing method. In this method a contrast medium is poured into a blood vessel of a brain to trace the momentary move of the medium to record the state of the entire cerebrovascular tract. The developed radiographic cerebrovascular image depicts the vascular tract on a high-density background; and in this instance, in order to find clearly the details of individual blood vessels, the shoulder portion (high-density region) of the characteristic curve of the radiographic image is required to be excellent in linearity as well as in sharpness.
In addition to the above problems, various light-sensitive materials may sometimes be desensitized by being subjected to various mechanical pressure prior to exposure (mechanical pressure prior to exposure causes desensitization that is recognized at the time of development). For example, medical X-ray film, since its size is large, tends to bend due to its own weight from its supported portion, thereby to forming film-bent troubles, so-called "knick marks," thereon, whereby pressure-desensitization troubles may occur. In recent years, mechanical transport system-applied automatic exposure/processing apparatus has been widely used as one of medical radiographic systems. In such apparatus, film is prone to be subjected to mechanical pressure, and particularly in a dry place in winter, pressure marks and pressure-desensitization troubles stated above tend to occur. And there is the possibility that such phenomena cripple medical diagnoses. Particularly, it is well known that the larger the grain size of and the higher the speed of a photographic light-sensitive material, the more does the light-sensitive material tend to form pressure-desensitization marks thereon. Examples of the use of thalium or dyes for the purpose of improving the pressure-desensitization problem are described in U.S. Pat. Nos.2,628,167, 2,759,822, 3,445,235 and French Pat. No. 2,296,204; and Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) Nos.107129/1976 and 116025/1975; but some of them show inadequate improvements, some show conspicuous dye stains, and some others can hardly be deemed to satisfactorily bring out the advantageous nature of silver halide photographic light-sensitive materials utilizing mainly the ordinary surface sensitivity of large-average-grain-size high-speed silver halide grains. On the other hand, many attempts have been made to solve the pressure-desensitization problem by changing the physical properties of the binder of silver halide photographic light-sensitive materials, as described in U.S. Pat. Nos.3,536,491, 3,775,128, 3,003,878, 2,759,821 and 3,772,032; and Japanese Patent O.P.I. Publication Nos.3325/1978, 56227/1975, 147324/1975 and 141625/1976. However these techniques, although useful for solving the pressure-desensitization problem, are unable to make substantial improvements because they cause deterioration of such binder's physical properties as the tackiness of the film surface, dryness, scratch resistance, and the like.
Measures for increasing the photographic speed, raising the covering power, improving the characteristic curve and sharpness, and solving the pressure-desensitization problem, described above, have hitherto been studied in various ways; but it has been very difficult to satisfy all these problems. For example, a technique that grows large silver halide grains for use in a silver halide photographic light-sensitive material to thereby raise the photographic speed thereof is well-known as a typical photographic speed-raising technique. However, if the grains are grown like this, the covering power deteriorates (according to the report by G. C. Farnell in The Journal of Photographic Science, 17, 116 (1969)).
If, on one hand, grains having large covering power are used with a low silver halide concentration to obtain a maximum optical density necessary for a photographic light-sensitive material, the result is lowering of the photographic speed thereof, which is not desired.
Thus, the attempts to raise both the photographic speed and the covering power are inconsistent.
The method for raising the photographic speed without changing the grain size, i.e., the sensitizing method, includes a large variety of techniques. If a proper sensitization technique is used, the speed can be expected to be raised with the covering power maintained, Various techniques of this kind are reported which include, e.g., methods of incorporating a development accelerator such as a thioether into an emulsion; methods of supersensitizing a spectrally sensitized silver halide emulsion in combination with appropriate dyes; techniques of improving optical sensitizers; and other equivalent techniques. These methods, however, are hardly considered widely usable for high-speed silver halide photographic light-sensitive materials. That is, the silver halide emulsion for use in high-speed silver halide photographic light-sensitive materials, when any of the above methods is applied thereto in order to make chemical sensitization to the utmost, tends to produce fog during the storage thereof. And in silver halide photographic light-sensitive materials for radiography use in which as small an amount of gelatin as possible is used in order to enable rapid processing, the above method deteriorates the resulting image quality.
Further, in the field of medical radiography, light-sensitive materials of the orthochromatic type, sensitive to 540-550 nm wavelength region, obtained by orthochromatically sensitizing conventional light-sensitive materials of the regular type, sensitive to around 450 nm, have come to be generally used. Thus sensitized light-sensitive materials are made so highly sensitive and have so wide a wavelength region to which they are sensitized that the exposure X-ray dose can be reduced to minimize the influence thereof upon the human body. The dye sensitization is thus very useful means for increasing the photographic speed, but there are many problems yet to be solved; for example, there are cases where no adequate photographic speed can be obtained, depending on the type of emulsions,--such problems still remain unsolved.
On the other hand, silver halide photographic light-sensitive materials (there are those of two types: one type having light-sensitive emulsion layers on both sides of the support thereof and the other having a single emulsion layer on one side; hereinafter called "radiographic light-sensitive material(s)," including both types) are required to have an excellent sharpness, large information capacity, excellent graininess, and to be hardly desensitized by pressure, and subject to little deteriorating in image quality.
For example, as for medical radiographic light-sensitive materials, the higher the sharpness and the better the graininess, the more easily can the diagnosis be performed; and the larger the information capacity, the more advantageous because diagnosability is higher; and it is desirable that the sensitive material be hardly desensitized even by pressure. Thus, all diagnostic information could be precisely turned into an image, which image could be excellent in preservability with no change in its quality.
Thus, in radiographing affected parts of a living body in the medical field and for the purposes of early detection of focuses and preventing wrong diagnoses, it is required that the radiographic image be so sharp and information capacity be so large that the image is highly diagnosable. Conventional radiographic light-sensitive materials, however, are not necessarily satisfactory in this respect.
Namely, conventional radiographic light-sensitive materials are classified into three types: as shown in the characteristic curves of FIG. 6, the high gamma type represented by the curve (a), the low gamma type by the curve (b) and the medium gamma type by the curve (c). However, the high gamma type (a), although highly sharp because of the steep rise of its characteristic curve, has a poor information capacity in the low exposure region. In contrast, the low gamma type (b) can be used in the curve (b') formed by shifting the curve (b) in parallel leftward through the control of the X-ray dose. In this instance since photographic density D in the low exposure region can be raised, the information capacity can be large, but the inclination of the characteristic curve is so gentle and therefore the sharpness is so low that it is hard to perform the diagnosis. And in the medium gamma type (c), the information capacity in the low exposure region as well as the sharpness is no more than moderate.
Typical gamma values with respect to the densities of the respective types of radiographic light-sensitive materials are as given in Table 1. In addition, regarding the respective gamas in the table, in the characteristic curves shown in rectangular coordinates wherein unit lengths on the axes of the coordinates formed with optical density (D) and exposure (log E) are equal to each other, the gamma formed between the optical density points 0.05 and 0.30 is regarded as gamma 1 (.gamma..sub.1), the gamma between the optical density points 0.50 and 1.50 as gamma 2 (.gamma..sub.2) and the gamma between the optical density points 2.00 and 3.00 as gamma 3 (.gamma..sub.3).
TABLE 1 ______________________________________ .gamma.1 .gamma.2 .gamma.3 D = 0.05- D = 0.50- D = 2.00- 0.30 1.50 3.00 ______________________________________ High gamma type 0.83-0.96 2.6-3.0 2.8-3.5 Medium gamma type 0.73-0.82 2.4-2.7 2.5-3.0 Low gamma type 0.68-0.72 2.0-2.2 1.2-1.5 ______________________________________
However, in the actual practice of radiographing with these conventional-type radiographic light-sensitive materials major problems occur. For instance, although the most frequently radiographed regions of living bodies in Japan are the chest, stomach and trabeculae of hands and feet, satisfactory radiographing of all of these regions cannot necessarily be carried out with the above-mentioned conventional radiographic light-sensitive materials.
First, referring to the chest, the important regions in reading a chest radiograph are the vascular tract in the lung field and the coronary artery behind the heart.
The lung field is a medium density region (D=1.3-1.5), and in order to read the vascular tract in the region, a high sharpness is necessary, while at the same time the coronary artery is in the low density region (D=0.05-0.30), so that a wide latitude is required; that is, it is essential that the exposure latitude be wide and adequate information be available from the image. However, conventional high gamma-type radiographic light-sensitive materials, although they can depict the lung field with high sharpness, depict the coronary artery only with very low density, so that they are substantially unable to contribute to diagnoses. In contrast, in the case where a low gamma-type radiographic light-sensitive material is used, although the coronary artery is depicted, the sharpness of the lung field region is poor.
Further, in the radiographing the stomach, since a contrast medium is used to improve its depiction, in conventional high gamma-type radiographic light-sensitive materials, when exposure is adjusted to the contrast medium's portion, the non-contrast-medium area, when developed, becomes solid black, thus making no contribution to diagnoses. In order to avoid such phenomenon, low gamma-type radiographic light-sensitive materials are mostly used. However, in the case of light-sensitive materials of this type, since the sharpness is lowered the diagnosability in the contrast medium-containing stomach wall region becomes deteriorated.
Also in the radiographing of trabeculae of hands and feet, etc., and soft parts (muscles or cartilage), conventional high gamma-type radiographic light-sensitive materials depict sharply the details of trabeculae, but make soft parts appear solid-black, thus failing diagnoses. In contrast, conventional low gamma-type radiographic light-sensitive materials depict sharply the soft parts, but show poor sharpness in the depiction of trabeculae.
And as for the pressure-desensitization problem of conventional radiographic light-sensitive materials, its improvement has been demanded as mentioned previously.